Microporous membranes, also referred to in the art as semipermeable membranes, are generally continuous structures, often in sheet form, of polymeric material with defined pore sizes. Depending upon the pore size, the membrane will retain bacteria, colloids, and particulates above a relatively small size; e.g. 0.1 micron (.mu.) in diameter. Such species are either retained on the membrane surface, or trapped in its substructure.
Microporous membranes are used in a wide variety of technical applications. For example, membranes with typical pore sizes of approximately 12 .mu. can be used for general clarification, filtration, and identification of large microorganisms. Membranes with smaller pore sizes such as 5 .mu. can be used in exfoliative cytology, chemotaxis, gravimetric amount analysis, gross particulate analysis of corrosive fluids, and cytologic evaluation of body fluids such as cerebrospinal fluid. Membranes with pore sizes such as 0.4 .mu. can be used for biological analysis of fluids, sterility testing, dewatering and purification of cellular suspensions, and immunology studies. As a final example, membranes with pore sizes as low as 0.1 .mu. or smaller can be used for filtration separation of viruses and proteins.
Other uses include humidification, chemical analysis, controlled release systems, electrochemical applications such as battery separators, capacitors and battery vents, industrial processes such as metal recovery, oil-water emulsion processing, protective clothing, ground water purification, packaging, liquid defoaming, fiber optics, composites processing, and information storage.
Other uses include "phase contacting" applications in which the membranes form the functional components of liquid-liquid extraction systems and gassing/degassing processes. For example, in liquid-liquid extraction, a membrane can help extract or transfer a component from one liquid to another without mixing the two liquids.
In yet other applications, microporous membranes form the functional core for sophisticated filtration systems, for supporting plant tissue cultures, for vacuum bag processing of structural composites, and for protective clothing. An example of this last category includes combat uniforms that can protect their wearers from chemical and biological warfare agents.
In almost all such applications, the membrane is placed against one surface, and usually between two surfaces, and serves the function of controlling the movement of some substance either to, from, or between those surfaces, depending upon the particular situation. As might be expected, there exists some circumstances in which the membrane and the surfaces are less compatible than would otherwise be desirable. Alternatively, the nature and function of the overall device structure may require that the membrane be bonded to one or both surfaces using an adhesive. In other circumstances, the membranes, which are often partially formed by a stretching step, are rather fragile in at least one direction, and can benefit from some sort of stabilization,
In any case, in order for the membrane to serve its intended purpose, the overall structure and composition of any device into which the membrane is incorporated should avoid interfering with the membrane's character and desired function.
An illustrative and widely growing use of microporous membranes is their incorporation in transdermal (through the skin) drug delivery systems; i.e. the transdermal patch. Generally speaking, transdermal drug delivery systems are used to deliver drugs to and through-the skin or mucosa of a wearer as a means of providing continuous, controlled administration of the drug. Transdermal delivery attempts to avoid the uncertainties of oral administration in which the pharmaceutical compound of interest may not be tolerated by the digestive tract, or in which larger dosages are given on a periodic basis in an attempt to have the body moderate the dosage between administrations. Related problems occur when injecting pharmaceuticals, compounded by the fact that most persons find needles unpleasant and may tend to avoid properly taking their doses on that basis, and that injections must often be given in a physician's office or other such setting.
A transdermal system attempts to avoid these problems by keeping a particular amount of a drug in a reservoir device having a size, shape and appearance similar to a common stick-on bandage. In addition to a drug-containing reservoir, a transdermal drug delivery system usually includes a rate controlling membrane on the side to be placed-against the patient's skin. Microporous membranes are ideal for this purpose. Such a device must also, however, include a means of attaching the membrane to the skin, and the usual technique is to use an adhesive. An adhesive, however, raises its own problems. For example, if the adhesive is incompatible with the microporous membrane, the attachment of the transdermal patch to the patient will be less than satisfactory, especially considering such devices are often worn continuously for a period of several days and must be maintained properly in place throughout the entire period in order to effectively deliver the proper dosage. It is also important that the adhesive remain on the surface of the membrane and that it not fill or block the membrane pores and thus hinder or prevent the delivery system from dispensing the necessary dosage.
Stated differently, the adhesive must keep the membrane in continuous contact with the skin. To do so, the adhesive must anchor properly to both the membrane and the skin without interfering with the membrane's function.
A number of techniques have been suggested for avoiding both problems. For example, in some devices the adhesive is applied to the microporous membrane in a pattern of coated and noncoated areas in order to leave some open areas of the membrane for the medication to pass through. Alternatively, in transdermal patches made in particular shapes, such as a circle, the adhesive is often applied around the perimeter of the patch (e.g. in concentric circles), in yet another attempt to provide the necessary adherence to the skin, while permitting the concurrent necessary transfer of medication through the membrane.
Such techniques raise a number of manufacturing difficulties, as well as generally less than satisfactory performance in the patches themselves. Accordingly, there exists the need for a method-of applying adhesive to a microporous membrane in-a-manner which is convenient, which. permits an appropriate adhesive to be anchored to the membrane, and yet which remains porous so that the membrane can effectively transmit the medication to the patient's skin.
Similar considerations apply to other uses of microporous membranes, such as fluid extraction, solid-liquid filtration, electrochemical applications and barrier fabrics such as in protective clothing.